The proposed research involves an analysis of the development and maintenance of the pair-bond and the ontogeny of parental care in the lion marmoset. Through the introduction of young females to males of different ages, the development of pair preferences is being studied. Long-established and newly-formed pairs will be compared to determine whether the duration of a monogamous relationship affects the quality or quantity of social interactions. The ontogeny of parental care will be analyzed by comparing the behavior toward infants by male and female juveniles and by adult pairs during their first and subsequent breeding episodes. During 1975-76 the development of pair preferences was observed in four trios, three consisting of a female and two males, and one of two females and a male. Observations on a total of 10 trios where pair bonds developed are still being analyzed. Although it currently appears as though females are choosing older, more dominant males, further observations of such trios are necessary to separate whether and how the age and dominance of the male are correlated. In any case, it is clear that, in a trio, one pair will emerge as bonded and restrict their sexual activity to each other, with the third animal being tolerated but not involved sexually.